Within this type of superconductors there exist several superconducting phases which differ in their number of copper-oxide layers in the crystalline unit cell. The composition of the superconducting phases of this system is generally described by the ideal formula Bi2Sr2Can-1CunOy where n represents the number of copper-oxide layers. For technical application the most interesting phases are those with n=2 and n=3, respectively, referred to BSCCO-2212 (also referred to “Bi-2221”) with an approximate transition temperature of 85 K and BSCCO-2223 (also referred to “Bi-2223”) with an approximate transition temperature of 110 K.
The unit cells of these phases have a layer structure consisting of BiO double layers that alternate with perovskite-like Sr2Can-1CunO1-2n units having also a layered structure with copper oxide sheets. As set out above, the Bi-2212 and Bi-2223 phases are characterized by 2 and 3 copper oxide sheets, respectively.
Generally, it is known, that some of the metal elements of these phases can be substituted, at least partially, by one or more other individual elements. For example, it is well known to substitute Bi partially by Pb. In particular, the Pb substitution of Bi leads to improved properties in case of the 2223 phase. Furthermore, calcium and strontium can substitute by each other.
In general, the high temperature superconductors of the BSCCO-family and methods for their production are well known in the art and numerous papers have been published regarding the discovery of these superconductors and the development of new methods including suitable starting compositions and heat treatments. For example, a summery of suitable methods is given in Superconductor, Science and Technology, Volume 6, number 1, January 1993 “Synthesis of cuprate superconductors”, pages 1 to 22 and in WO 00/08657 to which reference is explicitly made.
For example, in the so called ceramic route powdery mixtures of weight amounts of the starting materials such as the respective metal oxide, carbonates or other salts are formed so as to give the desired nominal composition, and the mixtures are homogenised and subjected to a suitable heat treatment for obtaining the desired superconductor.
For the heat-treatment the starting mixture can be calcined at a temperature of about 700 to 900° C. for a period of about 2 to about 200 hours. The calcined mixture is then ground, converted into the desired shape and sintered at temperatures of about 800 to about 1100° C. in the semi- or fully-molten state. In order to obtain a high quality superconductor material it can be preferably to perform the process stages of firing, such as e.g. calcining, sintering and optionally post-annealing, which may be carried out in a single firing operation or in several, in possibly even repeated, sub stages. Examples of suitable BSCCO-based compounds and production methods thereof are found, for example, in EP-B-0 330 305 and EP-A-0 327 044, to which reference is expressly made here.
It is also known to produce the desired superconductor body by melt casting processes. For the melt casting process the powdery starting mixture is melted, the melt is poured into moulds and allowed to solidify slowly therein. The solidified shaped body is removed from the mould and subjected to a heat treatment, for example at temperatures of 700 to 900° C., in an oxygen containing atmosphere, to obtain the final superconductor body. The principle of the melt casting process are described for example in DE-A-38 30 092 to which reference is expressly made here. According to this, parts of solid materials such as rods, plates etcetera can be obtained.
An expansion of the forming possibilities is shaping via centrifugal casting of the ceramic melt. The principles of this technique are described in DE-A-40 19 368 to which also reference is expressly made here. This technique is in particular suitable for superconductors of tubular and annular shapes. In the centrifugal casting the molten starting mixture having a predetermined stoichiometry is allowed to run at temperatures from 900 to 1100° C. into a rotating casting zone, for example a casting zone rotating about its horizontal axis. The solidified shaped body is removed from the casting zone and subjected to heat treatment for about 4 to 150 hours at about 700 to 900° C. in an oxygen containing atmosphere.
It was further known, to admix high melting alkaline earth sulfates with the powdery starting mixture for the BSCCO-superconductors such as SrSO4 and/or BaSO4. The sulfate can amount up to 20% by weight or in case of BaSO4, preferably, only up to about 10% by weight. Superconductors obtained by admixing such alkaline earth sulfates are disclosed for example in EP-A-0 524 442 and EP-A-0 573 798 to which reference is explicitly made.
Investigations by thermal analysis have revealed that the alkaline earth metal sulfates melt with the starting mixture, for example the oxides of Bismuth, Strontium, Calcium, Copper and, optionally, of lead, with the formation of eutectics. The melting point of Barium sulfate and Strontium sulfate are, specifically, considerably higher at 1580° C. and 1600° C., respectively.
Ceramographic investigations and photographs taken with the scanning electron microscope show that precipitations of Strontium sulfate and Barium sulfate are present in the high temperatures superconductors.
An insertion of the alkaline earth elements into the crystal structure of the unit cell of the superconductor phases, in particular, substitution of elements of the crystal structure of the unit cell by the alkaline earth elements has not been observed.
In U. P. Trociewitz et al. “The Influence of BaO2 additions on microstructure and superconductor properties of Bi2Sr2CaCu2O8+δ” Physica C000 (2001) pages 1 bis 13, BaO2 is admixed to the starting mixture and treated by the ceramic route. In the obtained superconductor body the influence of the addition of BaO2 on the development of texture in Bi-2212 is investigated. However, again, no substitution of elements of the unit cell by Ba is reported. Instead, it was observed that BaO2 reacted under formation of new second phases.
It is reported that addition of SrSO4 to the starting mixture and co-melting can reduce the risk of cracks of melt cast BSCCO 2212 (Elschner et al. “Influence of granularity on the critical current density in melt-cast processed Bi2Sr2CaCu2Ox” in Supercond. Sci. Technol. 6 (1993) 413-120).
In Tomochi Kawai et al. “Effekt of Ba addition on the properties of Bi—Pb—Sr—Ca—Cu—O superconductors” Japanese Journal of Applied Physics, volume 27, Number 12, December 1988, pages L2296 to L2299, BaCO3 is added to the powdery starting mixture of Bi2O3, PbO, SrCO3, CaCO3 and CuO and processed to a respective superconductor by sintering, that is by the ceramic route. It is stated, that the characteristic effect of the Ba addition seems to be the decomposition of the low Tc-phase of BSCCO superconductor by producing phases of BaBiO3 and BaCuO2 accompanied by high-Tc-phase formation. That is formation of second phases is observed rather than insertion or substitution of Ba into the crystal structure of the unit cell.
WO 97/49118 is directed to BSCCO material obtained by solid-state reaction wherein Bi may be partially substituted by a higher valance atom such as Pb, Hg, Re, Os etc. and Sr may be partially substituted by Ba or a larger lanthanide rare earth element. According to this publication due to the substitution the oxygen uptake of the Bi—O layer within the BSCCO structure is enhanced resulting in an increase of critical current density.
As set out above superconductors bodies with substantially round cross section such as cylindrical tubes, rods and annular bodies can be advantageously produced by the centrifugal melt casting process.
However, with the tubular superconductor components obtained by subjecting the prior art starting material to centrifugal melt casting irrespective of presence of SrSO4 cracks are observed which interrupt current flow.
Further, the obtained melt-cast shaped superconductor components are liable to bending during annealing which is particularly a problem when processing thin bodies such as thin tubes and rods etcetera. Due to these adverse effects homogeneity of the critical current over the superconductor body can vary to a substantial extent and deteriorate the performance of superconductor body.